Dynamic power and brightness control for a display screen

ABSTRACT

An image is displayed on an electronic display device at a reduced power level. Power used by the display device is maintained below a predetermined maximum power level by uniformly scaling the initial optical intensity of an image to a lower optical intensity whenever displaying the image at the initial optical intensity would result in power consumption of the display device exceeding the predetermined maximum power level.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of co-pending U.S. patentapplication Ser. No. 13/153,304, filed on Jun. 3, 2011, which claims thebenefit of India application number 1111/DEL/2011, filed Apr. 15, 2011,which claims benefit of U.S. provisional patent application Ser. No.61/352,297, filed Jun. 7, 2010. Each of afore mentioned patentapplications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

Embodiments of the present invention generally relate to displayscreens, and more specifically, to methods of dynamically controllingpower consumed by and brightness of display screens.

Description of the Related Art

Electronic display systems are commonly used to display information fromcomputers and other sources. Typical display systems range in size fromsmall displays used in mobile devices to very large displays, such astiled displays, that are used to display images to thousands of viewersat one time. Reduced power consumption is one desirable feature of suchdisplays, both because of the long-term energy savings provided to theuser and because of the reduced cost and complexity of installationassociated with systems having lower power requirements.

Some technologies for electronic display systems, such as laser phosphordisplays (LPDs) and organic light-emitting diodes (OLEDs), are able tosignificantly reduce power consumption by using a “color adding”approach to produce color at each pixel on the viewing surface.Specifically, red, green, and blue light energy is generated at a givenpixel to produce the desired brightness and hue for that pixel. Thus,the power use of LPDs, OLED displays, and the like is proportional tothe total optical energy produced by the viewing surface of the display.This is in contrast to display systems that produce color at each pixelon the viewing surface by selectively filtering or blocking light ofdifferent colors, such as a digital light processing (DLP) display. Insuch systems, white light source, such as an incandescent bulb, is setat full intensity at all times in such a display system. As such, thereis no reduction in power consumption when the system is producing darkerimages or images that do not require all three colors.

When averaged over a large number of images or over a relatively longtime interval, the power consumption of LPDs, OLED-based displayscreens, and other display systems that use a color-adding approach toproduce color can be substantially less than that of other displaytechnologies. However, such display systems provide less energy savingswhen most or all of a particular image being displayed is relativelybright. Thus, when displaying brighter images, less energy savings areprovided to the user by color-adding display systems.

In addition, in a tiled display system, power usage by the tiled displaysystem may exceed available power, when they are displaying brightimages. This may be true even in tiled systems employing LPDs. Upgradingthe power supply to such a system may be an option, but it can becostly. The other option is to recalibrate the system to a lower maximumpower level but this affects the quality of low brightness images andthe dynamic range.

As the foregoing illustrates, there is a need in the art for a method ofdisplaying an image with an electronic display device while stayingwithin the maximum available power.

SUMMARY OF THE INVENTION

One or more embodiments of the invention provide methods of dynamicallycontrolling the power consumed by, and brightness of images rendered on,an electronic display device. In one embodiment, power consumed by thedisplay device is reduced by uniformly scaling down the brightness ofthe rendered image in accordance with predefined settings so that powerconsumption of the display device stays within a predetermined maximumpower level.

A method of displaying an image on an electronic display system,according to an embodiment of the invention, includes the steps ofreceiving frames of image data to be displayed, determining an averagepower level for displaying one or more frames of image data, andadjusting a parameter of the electronic display system, such as a powerlevel or a brightness level, in accordance with the average power level.The average power level may be determined based on a single frame ofimage data or multiple frames of image data.

A method of displaying an image on a laser phosphor display devicehaving a plurality of laser sources, according to an embodiment of theinvention, includes the steps of receiving frames of image data to bedisplayed, determining an average power level for displaying one or moreframes of image data, and modulating the laser sources in accordancewith the average power level. The laser sources are modulated inaccordance with a plurality of maximum intensity settings, each of whichis set based on the average power level, and the maximum intensitysettings are predefined to increase as the average power leveldecreases.

A method of displaying images on a display device, according to anembodiment of the invention, includes the steps of receiving an inputrepresenting one or more images to be displayed, determining a displayvalue based on the input, comparing the display value against athreshold value, and adjusting a parameter of the display device basedon the comparison. In one embodiment, the display value is a power valueand the threshold value is a maximum allowed power value. In anotherembodiment, the display value is a display output intensity value andthe threshold value is a maximum allowed brightness value.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a block diagram of a display system that may benefit fromembodiments of the invention.

FIG. 2 is a schematic diagram of an exemplary laser-phosphor display(LPD) having a screen with phosphor stripes and a laser module that isused to produce one or more scanning laser beams to excite the phosphormaterial on the screen.

FIG. 3 is a partial schematic diagram of the portion of a screenindicated in FIG. 2.

FIG. 4 is a block diagram of a laser module in FIG. 2.

FIG. 5 illustrates one example of a 2×2 tiled display device that maybenefit from embodiments of the invention.

FIG. 6 is a graph illustrating a family of average power level (APL)mapping functions that may be used to define the dimming of an image tobe displayed by an LPD, according to embodiments of the invention.

FIG. 7 illustrates an APL mapping function having a smooth transitionbetween a constant value portion of the APL mapping function and adecreasing slope portion thereof.

FIG. 8 illustrates an APL mapping function having a relatively steepattenuation of the adjusted optical output that occurs well before theimage APL that corresponds to the power ceiling of an LDP.

FIG. 9 illustrates a family of APL mapping functions according to anembodiment of the invention.

FIG. 10 is a graph of desired optical intensity of a subpixel (in nits)as a function of optical output intensity of the subpixel or other lightsource (in DAC counts), where two gamma values are compared.

FIG. 11 is a flow chart that summarizes, in a stepwise fashion, a methodfor displaying an image on an electronic display device at a reducedpower level, according to embodiments of the invention.

For clarity, identical reference numbers have been used, whereapplicable, to designate identical elements that are common betweenfigures. It is contemplated that features of one embodiment may beincorporated in other embodiments without further recitation.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of a display system 100 that may benefit fromembodiments of the invention. Display system 100 may be used to displaya series of images, also referred to as frames, to produce a videosequence, or alternatively, a single static image. Display system 100includes a power source 101, a memory block 102, a controller 103, and adisplay screen 104. Power source 101 may be a conventional electricaloutlet, such as a 110 VAC or 220 VAC electrical outlet, a hard-wiredelectrical connection, or other electrical connection that provides therequisite voltage and amperage for the proper operation of displaysystem 100. Memory block 102 may include DRAM, flash memory, or othermemory devices for retaining image data 105 that is used to constructone or more images to be displayed by display system 100. Controller 103may include one or more appropriate processors for converting image data105 in memory block 102 to output signal 106, including general purposeprocessors such as micro-processors, digital signal processors (DSP),and special purpose processors, such as an application specificintegrated circuits (ASICs). Display screen 104 may be a laser phosphordisplay (LPD) screen, an organic light-emitting diode (OLED) baseddisplay screen, or other electronic display screen that generates acombination of different colors of light, e.g., red, green, and blue, ateach pixel to produce the desired brightness and hue for that pixel. Forexample, a pixel of display screen 104 may include a red, a green, and ablue (RGB) sub-pixel, which are used to generate the requisite red,green, and blue light that, in combination, produces the desired hue andbrightness for the pixel. In an LPD, such subpixels may be a phosphormaterial that is excited by a laser pulse. In an OLED-based displayscreen, such subpixels may be comprised of polymeric conducting andemissive layers positioned between an anode and a cathode. Displayscreen 104 may be based on other technologies as well, such as alight-emitting diode (LED) array.

In operation, display system 100 receives and stores image data 105 inmemory block 102. Image data 105 includes digital information forconstructing a single static image to be displayed by display system 100or a video sequence comprising a series of frames to be displayed bydisplay system 100. Image data 105 includes information such as opticalintensity of each subpixel of display screen 104 to produce the desiredimage or frame. Controller 103 extracts image data 105 for a singlevideo frame or static image from memory block 102, and calculates thetotal power required for display system 100 to display the frame orimage. Controller 103 then compares the calculated power to apre-determined maximum allowable power limit, i.e., a “power ceiling,”for display system 100. If the calculated power exceeds the maximumallowable power limit, controller 103 uniformly dims the frame or imageby scaling the brightness of each pixel and subpixel accordingly, sothat the total power used by display system 100 is below the powerceiling for display system 100 to produce the image. Controller 103 thensends output signal 106 to display screen 104, which produces the image.Output signal 106 includes the control signals required to produce theimage at a power below the maximum allowable power limit for displaysystem 100.

According to one or more embodiments of the invention, display system100 in FIG. 1 may be an LPD-based display system. FIG. 2 is a schematicdiagram of an exemplary LPD 200 having a screen 201 with phosphorstripes 202 and a laser module 250 that is used to produce one or morescanning laser beams 203 to excite the phosphor material on screen 201.Phosphor stripes 202 are made up of alternating phosphor stripes ofdifferent colors, e.g., red, green, and blue, where the colors areselected so that they can be combined to form white light and othercolors of light. Scanning laser beam 203 is a modulated light beam thatis scanned across screen 201 along two orthogonal directions, e.g.,horizontally 208 and vertically 209, in a raster scanning pattern toproduce an image on screen 201 for audience 206.

FIG. 3 is a partial schematic diagram of the portion of screen 201indicated in FIG. 2. FIG. 3 illustrates pixel elements 305, which eachinclude a portion of a red, green, and blue phosphor stripe 202. Theportion of the phosphor stripes 202 that belong to a particular pixelelement 305 is defined by the laser scanning paths 302, as shown. Lasermodule 250 forms an image on screen 201 by directing scanning laser beam203 along the laser scanning paths 302 and modulating scanning laserbeam 203 to deliver a desired amount of optical energy to each of thered, green, and/or blue phosphor stripes 202 found within each pixelelement 305. Each image pixel element 305 outputs light for forming adesired image by the emission of visible light created by the selectivelaser excitation of each phosphor-containing stripe in a given pixelelement 305. Thus, modulation of the red, green, and blue portions ofeach pixel element 305 control the composite color and image intensityat each image pixel element location.

In FIG. 3, one dimension of the pixel region is defined by the width ofthe three phosphor stripes 202, and the control of the laser beam spotsize defines the orthogonal dimension. In other implementations, bothdimensions of image pixel element 305 may be defined by physicalboundaries, such as separation of phosphor stripes 202 into rectangularphosphor-containing regions. In one embodiment, each of phosphor stripes202 are spaced at about a 500 μm to about 550 μm pitch, so that thewidth of pixel element 305 is on the order of about 1500 μm. In anotherembodiment, each of phosphor stripes 202 are spaced at a pitch betweenabout 125 μm and about 1000 μm.

FIG. 4 is a block diagram of laser module 250 in FIG. 2. Laser module250 includes a signal modulation controller 420, which modulates theoutput of a laser source 410 directly to control the energy delivered toeach of the phosphor stripes 202 found within each pixel element 305.For example, the signal modulation controller 420 may control thedriving current of a laser diode, which is found in the laser source410. A beam scanning and imaging module 430 projects the modulated beam,i.e., scanning laser beam 203, to screen 201 to excite the colorphosphors. Alternatively, laser source 410 is used to generate acontinuous wave (CW) un-modulated laser beam and an optical modulator(not shown) is used to modulate the generated CW laser beam with theimage signals in red, green and blue. In this configuration, a signalmodulation controller is used to control the optical modulator. Forexample, an acousto-optic modulator or an electro-optic modulator may beused as the optical modulator. In one embodiment, laser source 410further comprises two or more ultraviolet lasers 410A that are used inconjunction with other components in laser module 250 to deliver anarray of beams to the phosphor regions disposed on screen 201. Anexample of a laser based display system is further described in thecommonly assigned U.S. patent application Ser. No. 12/123,418, entitled“Multilayered Screens with Light-Emitting Stripes for Scanning BeamDisplay Systems,” filed May 19, 2008, which is incorporated herein inits entirety.

Tiled display walls provide a large-format environment for presentinghigh-resolution visualizations by coupling together the output frommultiple projectors. Such large displays may be created by tiling aplurality of smaller display devices together. For example, the videowalls frequently seen in the electronic media typically use multipleelectronic display devices, such as display system 100 or LPD 200, whichare tiled to create such large displays. Embodiments of the inventioncontemplate displaying an image on a tiled display wall device at areduced brightness and power level to avoid exceeding a predeterminedmaximum allowable power consumption by the tiled display wall. FIG. 5illustrates one example of a 2×2 tiled display device 500 that maybenefit from embodiments of the invention.

Tiled display device 500 includes a plurality of electronic displaydevices 510 mounted to a display frame (not shown for clarity). In theexample illustrated in FIG. 5, four electronic display devices 510 aremounted together in a 2×2 array. Other configurations of multipleelectronic display devices, e.g., 1×4, 2×3, 5×6, etc., may also benefitfrom embodiments of the invention. Each of electronic display devices510 is substantially similar in organization and operation to displaysystem 100 in FIG. 1, except that the electronic display devices 510 oftiled display device 500 are configured to operate in combination witheach other to display a single large format image or video sequence,rather than four independent images or video sequences. Tiled displaydevice 500 includes a main power supply 501, a memory block 502, and acentral controller 503. Main power supply 501 is configured todistribute electrical power to each of electronic display devices 510,and receives such power from a single point power source 520. Singlepoint power source 520 may be a conventional electrical outlet, such asa 110 VAC or 220 VAC electrical outlet, a hard-wired electricalconnection, or other electrical connection that provides the requisitevoltage and amperage for the proper operation of tiled display device500. Memory block 502 may include DRAM, flash memory, or other memorydevices for retaining image data 105 that is used to construct one ormore images to be displayed by tiled display device 500. Centralcontroller 503 is substantially similar to controller 103 in FIG. 1, butis further configured to separate image data 105 in memory block 502into output signals 506 and to direct each of output signals 506 to theappropriate electronic display device 510 so that a coherent image orvideo sequence is displayed by tiled display device 500.

In operation, tiled display device 500 receives and stores image data105 in memory block 502. Central controller 503 extracts image data 105for a single video frame or static image from memory block 502, andcalculates the total power required for tiled display device 500 todisplay the frame or image. Central controller 503 then compares thecalculated power to a pre-determined maximum allowable power limit,i.e., a power ceiling, for tiled display device 500. The predeterminedpower ceiling for tiled display device 500 is a user-defined or defaultquantity and may depend on a number of factors, including the maximumpower available from single point power source 520, a maximum desiredbrightness of tiled display device 500, etc. If the calculated power todisplay the image exceeds the power ceiling for tiled display device500, central controller 503 uniformly dims the frame or image by scalingthe brightness of each pixel and subpixel accordingly, so that tileddisplay device 500 uses a power level below the predetermined powerceiling to produce the image. Controller 503 then sends the appropriateoutput signals 506 to each electronic display device 510. Together, theelectronic display devices 510 produce the image or video frame at abrightness that ensures that the power ceiling of tiled display device500 is not exceeded. It is noted that the four portions of the imagecorresponding to each of the electronic display devices 510 are eachscaled down in brightness in the same way, so that the overallappearance of the image is uniform, and this is reflected in the datacontained in output signal 506 for each electronic display device 510.In some embodiments, the image brightness of one or more of theelectronic display devices 510 may be scaled down more than the others.This can be done by using a different set of mapping functions, the useof which are further described below.

According to one or more embodiments, calculating the total powerrequired for an electronic display device, e.g., display system 100, todisplay an image includes determining the average power level (APL) ofthe image to be displayed on the display screen of the device, e.g.,display screen 104. In terms of display system 100, the APL of an imageis defined herein as the ratio (expressed in percent) of the powerrequired by display screen 104 to produce the image at the powerrequired by display screen 104 to produce a fully white screen. Thus, ifdisplay screen 104 has 100 pixels (each with three RGB sub-pixels), andthe image to be displayed is made up of 10 fully white pixels, the APLof the image is 10%. APL quantifies the power required to display animage with respect to the maximum possible power draw of display screen104. In addition, because the power ceiling for display screen 104 canalso be quantified in terms of percentage of the total possible powerdraw of display screen 104, determining APL of an image allowscontroller 104 to quickly determine whether the brightness of an imageshould be reduced. Further, determining image APL facilitates the use ofAPL mapping functions, which, according to some embodiments, can be usedto define precisely how much an image should be dimmed.

In one embodiment, APL of an image is determined and APL mappingfunctions are then used to define if and how much the brightness of theimage is reduced in order to display the image with an electronicdisplay device while using less power than a predetermined power ceilingfor the display device. In terms of an LPD, such as LPD 200, for whichlaser input power is substantially equal to the optical intensity of theoutput, APL may be defined by the following equation:

${APL} = {\sum\limits_{i = 1}^{i = N}{I_{i}*\frac{1}{255*N}}}$

where N is the total number of subpixels of LPD 200 and I is theintensity of optical output of a subpixel (measured in DAC counts, i.e.,from 0 to 255). In this embodiment of APL determination, 0 DAC countscorresponds to no light generation by the subpixel and 255 DAC counts tomaximum light generation by the subpixel. It is noted that for otherelectronic display devices, the above definition of APL may requiremodification to compensate for a non-linear relationship between inputpower and output intensity and/or other inefficiencies of thelight-generating apparatus. Once the initial APL has been determined fora frame or image, a previously defined mapping function can be used toquantify the attenuation of each subpixel from an initial optical outputto an adjusted optical output, thereby reducing the power required todisplay the image below a pre-determined power ceiling.

FIG. 6 is a graph 600 illustrating a family of APL mapping functions601-605 that may be used to define the attenuation of an image to bedisplayed by an LPD, according to embodiments of the invention. In anexemplary embodiment used to describe the role of APL mapping functions,LPD 200 is an LPD-based display device having a power ceiling of 50 Wand a total possible output of 100 W when displaying a full white screenat 1000 cd/m² (also referred to as nits).

Each of APL mapping functions 601-605 defines the adjusted opticaloutput of a subset of subpixels as a function of image APL, where thesubset of subpixels includes subpixels having the same initial opticaloutput value. APL mapping function 605 describes the adjusted opticaloutput as a function of image APL for all subpixels forming an imagethat have an initial optical output of 255 DAC counts, i.e., maximumoptical output. Similarly, APL mapping function 603 describes theadjusted optical output for subpixels having initial optical outputs of200 DAC counts, APL mapping function 602 describes the adjusted opticaloutput for subpixels having initial optical outputs of 150 DAC counts,etc. For clarity, only five APL mapping functions are depicted in graph600. In practice, a large number of APL mapping functions may be used tospecify adjusted optical output, for example, one APL mapping functionmay be established for each initial optical output DAC count from 0 to255. Alternatively, fewer APL mapping functions may be established todefine adjusted optical output, e.g., one mapping function for every 10DAC counts, and an interpolation scheme may be used to determine anadjusted optical output of subpixels having an initial optical outputfalling between the established DAC count values. As shown, in theembodiment illustrated in FIG. 6, APL mapping function 601 remainssubstantially constant at 1000 nits when APL of an image to be displayedis less than or equal to 50%. For images determined to have APL greaterthan 50%, APL mapping function 601 decreases from 1000 nits when imageAPL is 50% to 500 nits when image APL is 100%. Similarly, the remainingAPL mapping functions 602-605 decrease in a corresponding fashion. Asnoted above, in this embodiment LPD 200 has a power ceiling of 50 W buta possible maximum power draw of 100W when displaying a full whitescreen, i.e., when all subpixels are at 1000 nits or 255 DAC counts.Thus, if LPD 200 were to display an image having an image APL greaterthan 50%, the power ceiling of 50 W would be exceeded if the subpixelsof LPD 200 display the image at the original optical output. Inspectionof FIG. 6 reveals that as image APL increases above 50%, the opticaloutput of each subpixel of LPD 200 is scaled down to an adjusted opticaloutput as indicated by APL mapping functions 601-605, so that the 50 Wpower ceiling of LPD 200 will not be exceeded and the relativebrightness and overall appearance of the image will not be altered.

The APL mapping functions 601-605 are only one example of such mappingfunctions contemplated by embodiments of the invention. The particularfeatures of the APL mapping functions, such as the slope of the mappingfunctions and the position of the inflection point of the mappingfunctions, depend on a number of factors, including the maximum possiblepower draw of LDP 200, the pre-determined power ceiling of LDP 200, andphysiological factors related to the human eye. For example, if themaximum possible power draw of LDP 200 is 1000 W and the power ceilingof LDP 200 is 300 W, then the scaling downward of APL mapping functions601-605 may occur at an image APL no greater than 30%, rather than at50% as shown in FIG. 6. In some embodiments, the power ceiling itselfmay be limited by external factors. For example, when power source 101is a standard 110 VAC, 20 A electrical outlet, the power ceiling of LDP200 cannot exceed the 2 kW rating of the electrical outlet. In anotherembodiment, the sharp inflection point 620, as shown in FIG. 6 for APLmapping functions 601-605, is contemplated in some embodiments, but inother embodiments a smooth transition may occur between the constantvalue portion of APL mapping functions 601-605 and the decreasing slopeportion thereof. FIG. 7 illustrates an APL mapping function 701 having asmooth transition 750 between a constant value portion 702 of APLmapping function 701 and a decreasing slope portion 703 thereof. Smoothtransition 750 may provide a more gradual transition in the dimming of avideo sequence, i.e., the increased dimming of a series of video framesover which APL continually increases. A more gradual transition can makesuch dimming of a video sequence less noticeable to a viewer. In someembodiments, the constant value portion of APL mapping functions 601-605as illustrated in FIG. 6 is reduced and/or eliminated. FIG. 8illustrates an APL mapping function 801 having a relatively steepattenuation of the adjusted optical output that occurs well before theimage APL 802 that corresponds to the power ceiling of LDP 200.

Other APL mapping curves are also contemplated by embodiments of theinvention. In some embodiments, when most of an image has relatively lowbrightness and a small percentage of the image has brighter pixels, anAPL mapping function is used in which the adjusted optical output of thebrighter subpixels is increased. In such an embodiment, the brightestsubpixels making up the image are scaled upward in brightness, so thatthese brighter subpixels may have an adjusted optical output that ishigher than their initial optical output. Thus, for an image having verylow APL values, e.g., under about 20%, the adjusted optical output ofsubpixels in the image that are at or near 100% output are increased inbrightness. Images produced by such an embodiment can be qualitativelymore pleasing to the human eye than an image in which the small numberof bright pixels are scaled downward from 100% optical output. FIG. 9illustrates a family of APL mapping functions 900 according to such anembodiment of the invention. APL mapping function 901 specifies, as afunction of image APL, the adjusted optical output of subpixels in animage having 100% optical output, where 300 nits is defined as 100%optical output. APL mapping function 902 specifies the adjusted opticaloutput of subpixels in an image having 60% optical output, and APLmapping function 903 specifies the adjusted optical output of subpixelsin an image having 20% optical output. As shown, for low-APL images, thebrighter subpixels are increased in intensity to a brightness valuegreater than the nominal maximum optical output for the subpixel, whichin this example is 300 nits. Because the adjusted optical output of onlya small number of pixels is increased in this way, and because such anadjustment occurs only when image APL is relatively low, the powerceiling for LPD 200 will not be violated in such an embodiment.

In some embodiments, an electronic display device may use different APLmapping functions at different times. For example, a reduction in totalavailable electrical power for an electronic display device may resultin a different power ceiling for the display device. Because APL mappingfunctions are based in part on the power ceiling for a display device,different APL mapping functions may be used by the display device fordifferent available power scenarios. Similarly, a user may selectdifferent APL mapping functions as ambient light conditions change. Forexample, maximum brightness of the adjusted optical output subpixels maybe modified based on ambient light conditions. Alternatively, theoverall shape of the APL functions being used may be altered as ambientbrightness changes, in order to better satisfy the physiological needsof the human eye.

As described above, image APL may be calculated and APL mappingfunctions applied to a static image or an individual frame of a videosequence to generate adjusted optical output values for each subpixel ofthe image or frame. In order to provide more gradual dimming orbrightening of the frames making up a video sequence, in someembodiments image APL is calculated based on a plurality of frames. Forexample, image APL may be a time average of multiple frames, e.g., theten most recent frames in a video sequence. In such an embodiment,dimming or brightening of a video sequence may be less noticeable to aviewer. The number of frames that is time averaged may be selected inview of the physiological needs of the human eye (e.g., to reduce eyefatigue in video sequences where the image APL fluctuates at a highrate).

Embodiments of the invention further contemplate maintaining a constantvalue for gamma correction when attenuating the brightness of an imagefrom an initial optical output intensity to an adjusted optical outputintensity. By holding gamma correction constant when the APL of areceived image is reduced prior to display, the image will not bealtered significantly in appearance. Gamma correction, often simplyreferred to as “gamma,” is a nonlinear operation used to code and decodeluminance in video or still image systems. In order to maintain constantgamma for an image when the image brightness is attenuated according toembodiments of the invention, e.g., using a family of APL mappingfunctions, the adjusted optical output intensity for each subpixel ismodified accordingly as a function of gamma. FIG. 10 is a graph 1000 ofdesired optical intensity of a subpixel (in nits) as a function ofoptical output intensity of the subpixel or other light source (in DACcounts), where two gamma values are compared. In the example illustratedin FIG. 10, a setting of 255 DAC counts for the subpixel produces amaximum output of 1000 nits and a setting of 0 DAC counts producesessentially no optical output, or 0 nits. Graph 1000 includes function1001, which illustrates the desired optical intensity vs. DAC countswhen gamma equals 1.0, and function 1002, which illustrates the desiredoptical intensity vs. DAC counts when gamma equals 2.2.

For fully saturated images, gamma is typically equal to 1 and there is alinear relationship between DAC counts applied to a subpixel and thedesired optical intensity of the subpixel. Thus, when an APL mappingfunction specifies that an initial optical output value 1010, e.g., 1000nits, should be reduced to an adjusted optical output value 1020, e.g.,500 nits, the DAC counts controlling the output intensity of thesubpixel are reduced proportionally from 255 DAC counts to 128 DAC,since 255*(500/1000)=128. In video sequences, gamma is generally setequal to 2.2 for reduced eye strain and a more natural-appearing image.As shown in graph 1000, there is a non-linear relationship between DACcounts applied to a subpixel and the desired optical intensity of thesubpixel, which reflects the more readily perceived differences in lowerintensity images by the human eye. Thus, when an APL mapping functionspecifies that initial optical output value 1010 should be reduced toadjusted optical output value 1020, the DAC counts controlling theoutput intensity of the subpixel are reduced from 255 DAC counts to 186DAC counts (instead of 128 DAC counts). Similarly, other DAC countvalues for a subpixel can be determined from the relationship depictedin FIG. 10 by function 1002 when a gamma of 2.2 is to be maintained whenattenuating the subpixels of an image to an adjusted optical output.

In embodiments of the invention in which the value of gamma isconsidered when attenuating the brightness of an image from an initialoptical output intensity to an adjusted optical output intensity, imageAPL may be determined using the following equation:

${APL} = {\sum\limits_{i = 1}^{i = N}{I_{i}^{\gamma}*\frac{1}{255*N}}}$

where N is the total number of subpixels of LPD 200 and I is theintensity of optical output of a subpixel (measured in DAC counts, i.e.,from 0 to 255).

According to some embodiments of the invention, a tiled display, such astiled display device 500 in FIG. 5, may also use APL mapping functionsto define if and how much the brightness is reduced for an image to bedisplayed by the tiled display. In such embodiments, the image isdisplayed using less power than a predetermined power ceiling for thetiled display device. The predetermined power ceiling may be based onthe available power for the tiled display device. For example, whensingle point power source 520 of tiled display device 500 is a standard110 VAC, 20 A electrical outlet, the power ceiling of tiled displaydevice 500 cannot exceed the 2 kW rating of the electrical outlet. Insuch embodiments, the APL of the entire image is calculated, rather thanthe APL for each individual electronic display device mounted togetherto form the tiled display device. Further, APL mapping functions usedare applied uniformly to all pixels and subpixels of the tiled displaydevice regardless of which particular electronic display device asubpixel is an element of. The same APL mapping functions are applieduniformly to all subpixels of all electronic display devices to preventnon-uniform attenuation of an image.

FIG. 11 is a flow chart that summarizes, in a stepwise fashion, a method1100 for displaying an image on an electronic display device at areduced power level, according to embodiments of the invention. By wayof illustration, method 1100 is described in terms of an LPD-basedelectronic display device substantially similar to LPD 200 in FIG. 2.However, other electronic display devices may also benefit from the useof method 1100. Prior to the first step of method 1100, a user-selectedpower ceiling is selected. Such a power ceiling may be determined basedon a variety of factors, including the capacity of power source 101.Based on the power ceiling for LPD 200 and the maximum possible poweruse of LPD 200, which in a multi-tiled display depends on the totalnumber of display tiles, a family of APL mapping functions isconstructed.

In step 1101, LPD 200 receives and stores image data 105 in memory block102. Image data 105 includes digital information for constructing asingle static image to be displayed by LPD 200 or a video sequencecomprising a series of frames to be displayed by LPD 200. Image data 105includes information such as the required intensity of each subpixel ofscreen 201 to produce the desired image or frame. In some embodiments,image data 105 includes digital information for constructing a pluralityof frames in a video sequence.

In step 1102, controller 103 extracts image data 105 for a single videoframe or static image from memory block 102, and calculates the totalpower required for display system 100 to display the frame or image.

In step 1103, controller 103 compares the calculated power to thepre-determined power ceiling for LPD 200. If the calculated powerexceeds the power ceiling, controller 103 uniformly dims the frame orimage by scaling the brightness of each pixel and subpixel to anadjusted optical output intensity. A family of APL mapping functions maybe used to determine the adjusted optical output intensity for eachsubpixel as a function of image APL and of the initial optical output ofthe subpixel. In some embodiments, gamma correction is considered whenattenuating the brightness of an image from an initial optical outputintensity to an adjusted optical output intensity. Consequently, thevalue of gamma for the attenuated image will be substantially the sameas the value of gamma for the original image, thereby minimizing visualartifacts noticeable by a viewer.

In step 1104, controller 103 sends output signal 106 to screen 201,which produces the image. Output signal 106 includes the control signalsrequired to produce the image at a power below the maximum allowablepower limit for LPD 200, including the modulation signals for eachsubpixel in screen. In embodiments in which LPD 200 is a multi-tileddisplay, controller 103 also divides the attenuated image into aplurality of separate images, each of which is sent to the appropriatedisplay tile. In such embodiments, output signal 106 is a differentsignal for each display tile making up the multi-tiled display.

In step 1105, screen 201 displays the attenuated image based on outputsignal 106.

In sum, embodiments of the invention contemplate methods of displayingan image with an electronic display device to produce an image of adesired size and at a lower power than other display systems. Suchmethods provide long-term savings in energy costs to the user. Inaddition, by reducing the maximum power draw of the display device, suchmethods allow relatively large displays to operate using a standard, andoften pre-existing power source. For example, by using embodiments ofthe invention, a relatively large display system may operate from astandard 110 VAC electrical outlet, thereby providing maximumflexibility in installation of the display and avoiding the complexityof installing specially sized wiring, circuit breakers, etc. Further,because embodiments of the invention lead to, on average, substantiallylower power output for laser light sources that may be used as lightsources, the lifetime of such lasers is significantly extended. Lastly,the user-defined power ceiling and APL mapping functions provideflexibility in performance of a display system that can be optimized bythe user based on changing power availability, ambient lightingconditions, and so on.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

We claim:
 1. A method of displaying an image on an electronic displaysystem, comprising: receiving frames of image data to be displayed,wherein the image data includes an optical output intensity of eachpixel and subpixel of a display device of the display system to producethe image or frame and the sub-pixels are red, green, and bluesubpixels; determining an average power level for displaying one or moreframes of image data, wherein the average power level is defined as theratio of the power required by the display device to produce an image atthe power required by the display device to produce a fully whitescreen; determining an adjusted optical output intensity for each pixeland subpixel as a function of the average power level and the opticaloutput intensity of each subpixel; adjusting a parameter of theelectronic display system in accordance with the average power level,wherein adjusting a parameter comprises scaling the brightness of eachpixel and subpixel to the adjusted optical output intensity; and whereinthe electronic display system includes multiple display devices eachwith a light source and wherein each light source produces a portion ofthe image data.
 2. The method of claim 1, wherein the average powerlevel is determined based on a single frame of image data.
 3. The methodof claim 1, wherein the average power level is determined based onmultiple frames of image data.
 4. The method of claim 3, wherein theparameter is continuously set each time a frame of image data isdisplayed, based on the average power level that is determined from arunning average of the multiple frames of image data.
 5. The method ofclaim 1, wherein said adjusting includes scaling down a brightness levelof the electronic display system.
 6. The method of claim 5, wherein thebrightness level of the electronic display system is scaled down inaccordance with a function that maps target brightness levels to averagepower levels.
 7. The method of claim 1, wherein said adjusting includesscaling down a power level of the electronic display system.
 8. Themethod of claim 7, wherein the power level of the electronic displaysystem is scaled down in accordance with a function that maps targetpower levels to average power levels.
 9. The method of claim 1, whereinthe parameter is adjusted so as to maintain a desired gamma.
 10. Themethod of claim 1, wherein the average power level is defined by thefollowing equation:${APL} = {\sum\limits_{i = 1}^{i = N}{I_{i}*\frac{1}{255*N}}}$ where Nis the total number of subpixels of the display device and I is theintensity of optical output of a subpixel (measured in DAC counts).